Development and Application of Slow Crack Propagation of Polyethylene Based on Crack Layer Theory

Abstract:

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For explaining the SCG behavior of polyethylene, the crack layer theory is applied based on
the description of two driving forces: crack and PZ. The relations between the speed of SCG, crack
length and elapsed time are the most important characteristics of polyethylene resistance to crack
propagation, or long-term brittle fracture. The crack layer model of slow crack growth in polyethylene
is designed in such a way that it qualitatively reproduces the main features of the process indicated
above and makes it possible to quantitatively match any pattern of step-wise crack growth. In this
paper, the behavior of SCG of polyethylene is developed for numerical simulation based on the crack
layer theory. Some parametric study and applications are addressed based on the developed
simulation program.

Abstract: A two-parameter constraint-based fracture mechanics approach is used to explain the effect of the constraint on the apparently anomalous behavior of short fatigue cracks. The different levels of stress constraint are quantified by the T-stress, and microstructurally as well as mechanically short cracks are discussed. Short cracks generally behave more sensitively to the constraint than the long ones. It is shown that in most cases, the existence of short cracks goes hand in hand with an intrinsic loss of the constraint, which contributes to a decrease of their fatigue
threshold values and accelerates their growth. In this paper, the above effect is quantified and conclusions concerning the applicability of the fracture mechanics parameters and approaches to the estimation of the residual fatigue life of structures are discussed.

Abstract: In-situ SEM observations have revealed that fatigue crack propagation in aluminium
alloys is caused by the shear band decohesion around the crack tip and the formation and cracking
of the shear band is mainly caused by the plasticity generated in the loading part of the load cycle.
This shear band decohesion process has been observed to occur in a continuous way over the time
period during the load cycle. Based on this observation, in this study, the transient fatigue crack
growth rate, da/dt, has been used to obtain the relationship between the conventional used parameter
da/dN and the applied driving force. It is proven that two parameters are necessary in order to
accurately describe fatigue crack propagation rate per stress cycle, da/dN. The well known stress
ratio effects on fatigue crack propagation rate can be correlated by this model.

Abstract: Fatigue crack growth under mixed mode loading conditions is simulated using S-FEM. By
using S-FEM technique, only local mesh should be re-meshed and it becomes easy to simulate crack
growth. By combining with auto-meshing technique, local mesh is re-meshed automatically, and
curved crack path is modeled easily. Plural fatigue crack problem is solved by this technique. For
two parallel crack problem, criteria of crack coalescence are proposed. By simulating this problem by
S-FEM, it is verified these criteria are conservative ones.

Abstract: In this paper, the mechanisms of propagation of the damage in aluminum panels repaired with bonded composite patches of different mechanical characteristics is analyzed. The aim of this study is to analyze analytically, experimentally and numerically the advantage of the use of bonded composite patches to increase the fatigue life and to reduce the state of tension at the crack tips. The experimental results show that both static strength and fatigue life of the repaired aluminum panel has significantly increased due to the bonded composite patches. The different patches and adhesive, used for cracked panels, have provided about a 100-110% improvement in the fatigue life and a 30-35% decrease in the stress intensity factor. A comparison between finite elements calculations and experimental data has been carried out. The good agreement between the experimental data and the numerical ones has demonstrated the possibility to obtain an optimized design of bonded patches with the numerical tools.

Abstract: The wedge splitting (WS) test is now a promising method to perform stable fracture mechanics tests on concrete-like quasi brittle materials. Fracture parameters, such as fracture toughness and critical crack opening displacement and et.al, are however not easy to determined since formulae available from stress intensity factor manual are restricted to standard specimen geometry. The paper attempts to compute expressions for commonly used fracture parameters for a general wedge splitting specimen. By means of finite element analysis program, test simulation was performed on non-standard wedge splitting specimen with different depth and initiation crack length, and thereafter expressions were proposed for stress intensity factor at the pre-cast tip and crack mouth opening displacement on the load line. Based on the work above, size effect on the unstable fracture toughness and crack extension were investigated, and the consistency of fracture toughness data for various specimen depth as well as initiation crack length is demonstrated. The crack extension is little sensitive to the initiation crack length, it increases with the depth of specimen, which can be explained by the boundary influence of the specimen.